Seminars Archive

4D X-ray microtomography of bubble growth in basaltic magmas: implications for volcanic eruptions

Abstract
Understanding the influence of bubble size distributions on magma permeability and strength is critical to investigations of volcanic eruption mechanisms. As bubbles grow the walls between them thin to where they may no longer have enough mechanical strength to hold expanding magmatic foams together, resulting in fragmentation and eruption, but if foam permeability increases sufficiently rapidly gases may leave the system and lessen the hazard. Using a laser-heated, high-temperature furnace on the TOMCAT beamline of the Swiss Light Source we were able to perform X-ray microtomography on growing bubbles in a silicate melt foam for 18 s at a rate of 1 Hz. The collected data were then quantitatively analysed by using the Pore3D software library custom-developed at Elettra. We measured the geometrical properties of the volcanic foam and used them to model the transport of gas through the foam and the conditions under which it fails. In particular we determined bubble size and wall thickness distributions, as well as connectivity, and calculated permeabilities and tensile strengths of basaltic foams imaged by ultra-fast synchrotron X-ray microtomography during bubble growth in basaltic melts with either 3 or 7 wt. % H₂O at 1 atm. In this seminar we will show that rapid vesiculation leads to highly porous foams whose tensile strengths are less than 20% of the bulk and whose permeabilities increase from ~ 1 x 10^-10 to 1 x 10^-9 m² between 10 and 14 s despite decreasing connectivity between bubbles. Tensile strength decreases more rapidly than permeability increases, suggesting that basaltic magmas are most susceptible to failure immediately upon vesiculation and that at later times, perhaps only 10's of seconds later, permeability increases may lessen the hazard of a violent eruption. A four dimensional X-ray tomographic microscopy study of bubble growth in basaltic foam, D.R. Baker, F. Brun, C. O'Shaughnessy, L. Mancini, J. Fife & M. Rivers, Nature Communications, 3 (2012) 1135, doi: 10.1038/ncomms2134.